D-xylose can be recovered from a variety of materials after hydrolysis of xylans to xylose. There are many prior art processes which claim to provide acceptable means for obtaining xylose from natural materials, such as birch wood, corn cobs and cotton seed hulls, among others.
One method of recovery of xylose from the aforementioned materials is by chromatographic separations, such as that discussed in U.S. Pat. No. 4,075,406. Therein, the raw material is hydrolyzed and then purified by ion exchange and color removal. The ion exchange is accomplished by passing the solution through successive beds of strong cation exchanger and weak anion exchanger. The purified solution is then subjected to chromatographic fractionation to provide a solution containing xylose. The resin employed in the chromatographic separation is a strongly acid cation exchanger, sulfonated polystyrene cross-coupled with 3.5% divinyl benzene, the resin being in calcium form.
Anion exchange resins have been used in the past for separating fructose from glucose. Y. Takasaki (Agr. Biol. Chem. 36 (1972) pages 2575-77) and B. Lindberg et al. (Carbohyd. Res. 5 (1967), pages 286-291) describe the use of an anion exchanger in bisulfite form for the separation of sugars.
The use of anion exchange resins has been found to be disadvantageous, however, because xylose is eluted between other sugars and not separated last. This results in a poor xylose separation.
Anion exchange resins in the sulfate form have been tested by Samuelson et al. (Acta Chem. Scand. 22 (1968), pages 1252-58). In these tests, ethanol was used as the eluent Solution rather than water. The resultant xylose separation was said to be unsatisfactory, and cation exchange resins were recommended instead for sugar separation.
The separation of xylose by cation exchangers has been practiced industrially but is complicated. This method of separation requires two steps. In the first step, ionized substances and high molecular weight substances are separated from low molecular weight substances by ion-exclusion. In a second step, a xylose-rich solution is recovered after chromatographic separation of the sugars. In both separation steps, a cation exchange resin is used. In the ion-exclusion step, the resin is preferably in alkali-metal form (e.g., sodium or potassium). In the second step, the resin is in alkaline earth form (e.g., calcium or strontium). The separation of xylose by this method has also been found to be unsatisfactory.
It is therefore an object of the present invention to provide a method for obtaining an excellent separation of a xylose-containing solution from a raw material.
It is another object of the present invention to provide a method for the recovery of xylose from a xylose-containing solution which is uncomplicated and which can be practiced commercially.
It is another object of the present invention to provide a method for the recovery of xylose from a xylose-containing solution which requires a single separation step.
It is still another object of this invention to separate xylose from all the other monosaccharides by single step.